During processing of silver halide photographic elements, the developer is oxidized to a silver salt by a suitable bleaching agent. The oxidized silver is then removed from the element in a "fixing" step.
The most common bleaching solutions contain complexes of ferric ion and various organic ligands. One primary desire in this industry is to design bleaching compositions which are more compatible with the environment, and thus it is desirable to reduce or avoid the use of ferric ions and many of the common complexing ligands.
Persulfate bleaching solutions offer an alternative to the ferric complex bleaching solutions. However, persulfate bleaching agents are slow in bleaching performance unless they are used with bleach accelerators. Most commonly used accelerators are thiols which often have objectionable odors and are unstable when incorporated directly into bleaching solutions.
Other accelerators are known. For example, U.S. Pat No. 3,748,136 (Willems) describes the use of aromatic amines to catalyze persulfate bleaching agents. Two other publications, DD 0141727 and DE 3,234,467, describe the presence of a quinone in persulfate bleaches, and optionally the presence of cupric ion. The inherent stability of these catalysts in the presence of persulfate, however, is poor.
There remains a need, therefore, for highly efficient persulfate bleaching solutions which do not suffer from the problems noted above, that is, they are rehalogenating, stable and useful for a variety of photographic elements, and lack objectionable odors.